[EKS] [request]: Adding subnets from previously unused AZs

[blanchardma]

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Tell us about your request
Add subnets of Availability Zones (AZs) to the EKS control plane that were not originally provided when the cluster was created. OR IN OTHER WORDS: We need to expand our EKS to include subnets from additional Availability Zones (AZs) that weren't originally included when the cluster was set up. (For a better context read also the limitation at the end.)

Which service(s) is this request for?
EKS

Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?
The ability to extend the number of Availability Zones (AZ) and corresponding subnets for the EKS control plane, so that elastic network interfaces (ENIs) of the EKS control plane can also be created in the customer VPC.
This is difficult because it is not supported by the AWS service and you have to recreate the entire EKS control plane.

Are you currently working around this issue?
Not really, you can either recreate the EKS control plane or leave it in the previous subnets and AZs.

Additional context
The limitation is currently described as follows in the AWS documentation:
"All subnets that you add must be in the same set of AZs as originally provided when you created the cluster."
https://docs.aws.amazon.com/eks/latest/userguide/network_reqs.html#network-requirements-vpc